The present invention is in the field of phase-change thermal energy storage (TES) and is specifically directed to a system in which a plurality of containers each containing a phase-change material (PCM), that is, a latent heat accumulating fluid, are arranged in a tank in a unique self-supporting manner that allows the associated thermal transfer fluid to operate at a high volumetric efficiency.
Thermal energy storage (TES) is a well-known technology for cooling buildings during daytime on-peak periods when the cost of maintaining the primary modes of cooling are at their highest. In order to cut these high-energy costs during peak periods, thermal energy can be stored during off-peak or low-demand periods and then used during the peak, high-cost periods. During off-peak periods (nighttime, for example) a coolant, a readily available building coolant, for example, is circulated around the storage system at below freezing temperatures in order to freeze the liquid contained in the containers. The ice formed in the containers is thus stored, and the containers are subsequently exposed to a thermal transfer fluid for providing low-cost refrigeration during a peak daytime period. Such storage systems normally employ a number of different methods to effect thermal storage. For example, one such method is to circulate a readily available building coolant through a number of pipes positioned within a liquid-filled tank to cause ice-formation on the exterior of the pipes. The liquid in the tank becomes cooled and then can be used as a thermal transfer fluid as needed. This method, however, requires extensive monitoring equipment to insure that the ice-build up around the pipes does not become unmanagable. In another method, plate-ice is formed by spraying water on plate coils which are alternately cooled to form ice and warmed to release the ice which is stored in a liquid-filled tank. This method requires an elaborate cooling and heating mechanism as well as additional storage facilities apart from the ice-formation facility.
In yet another method, to which the present invention relates, semi-rigid containers having a phase-change medium (PCM) therein are stored in a tank. In one such system a gas fluid, such as air, operates as a thermal transfer fluid and is circulated around the stacked containers which have stand-off projections cast intergrally thereon to allow the fluid to flow between them. Each container in this system is filled with a PCM, such as water, or a latent heat storage material, such as polyethelene glycol or calcium chloride hexahydrate. In this particular system stand-off projections on the individual containers can serve to both support the superimposed containers and create, as well, the flow-through paths between the top and bottom surfaces of the containers. While this arrangement may be effective for a gas thermal transfer fluid, such as air, it is ineffective for a liquid fluid which is used as a thermal transfer medium. Also, a gas thermal transfer fluid has low specific heat and density and is therefore a poor thermal transfer medium. In liquid TES systems using stacked containers, support structures, such as rods and shelves are normally employed to support the stacked containers in the tank. This additional support structure, often constructed of metal and subject to corrosion, becomes an unwanted obstacle then, to the economic use of space. Additonally, the containers presently used are only partially filled with a PCM in order to allow for expansion of the liquid within the container. Often, in such thermal storage systems the PCM occupies 30 to 65% of the system's volume, and for that reason inefficiently low refrigeration temperatures are needed to form ice in the containers.
There is a need, therefore, to provide a TES system which uses a liquid thermal transfer fluid in a tank that can operate with increased volumetric efficiency, improved thermal transfer, efficent use of space, and which makes use of an efficient individual container design for storing a PCM.